The present invention relates to aqueous emulsion polymers, to their use as surface sizing agents, and to processes for preparing the emulsion polymers.
The prior art has disclosed sizing agents based on aqueous emulsion polymers, but these are not satisfactory with respect to all of the requirements made of them.
Japanese Patent Application 58/115196 describes the use of aqueous dispersions of graft copolymers of acrylates and styrene on starch as sizing agents for paper, and European Patent Applications 257,412 and 276,770 claim graft copolymers of acrylates and acrylonitrile on starch which are likewise employed in the form of fine aqueous dispersions for sizing paper.
European Patent Application 276,770 also describes how, by adding from 0 to 3.0% by weight, based on the sum of the monomers employed, of low molecular mass emulsifiers such as sodium alkyl-sulfonate, sodium lauryl sulfate, or sodium dodecylbenzenesulfonate, it is possible to stabilize the emulsions claimed therein. However, a disadvantage is said to be that adding such an emulsifier impairs sizing and that the sizing agents are often associated with severe foaming.
Alternatively, European Patent Applications 331,066 and 400,410 disclose fine dispersions of sizing agents that have excellent performance properties and are obtained by emulsion polymerization of monomers such as acrylonitrile, butyl acrylate, or styrene, in the presence of polymeric anionic emulsifiers containing sulfonic acid groups (in the case of European Patent Application 331,066, based on maleic anhydride copolymers, and in the case of European Patent Application 400,410 based on polyurethanes with stellate branching). Sizing agent dispersions of this kind exhibit excellent performance properties, especially on neutral and chalk-containing papers. However, such dispersions have low stability to divalent and trivalent cations, such as Ca.sup.2+ or Al.sup.3+. Under adverse conditions, this may sometimes result in precipitation on the sizing press and thus to a deterioration in their sizing effect.
It has surprisingly now been found that emulsion polymers based on starch graft copolymers and polymeric emulsifiers are of outstanding suitability as sizing agents that couple improved performance properties with high dispersion stability to divalent cations and an extremely low tendency to foam in use.